BEIRUT — Rebel fighters said Monday that they shot down a MiG-23 jet in eastern Syria and captured the pilot, a claim disputed by the Syrian government, which blamed a “technical failure” for the crash.

If the rebel assertion is verified, the action would represent a significant improvement in the military skills demonstrated by the ragtag fighters and could be a sign that they are receiving more sophisticated weapons from their international sponsors, which now include Qatar and Saudi Arabia.

The jet apparently went down in Mohassan, a small town southeast of Deir al-Zour, a flash point in the battle between opposition forces and President Bashar al-Assad’s government.

The official Syrian Arab News Agency posted an article that said the jet crashed because of technical reasons and that a search for the pilot was underway. “A technical failure that happened to a military aircraft while it was on an ordinary training flight over the eastern area caused the command devices to break down, and the pilot to leave the plane by the ejection seat,” said a military source quoted in the article.

Aref Hammoud, an FSA spokesman based in Turkey, said Monday that the jet was shot down with a 14.5 mm anti-aircraft gun, not missiles. “Machine guns were used to shoot at the plane. It was in a low range, which made it possible to hit,” he said. “Those machine guns were captured from the regime army and God helped us to hit down this plane.”

The downing of the jet, perhaps the first by rebel forces, shows that the fighting is ramping up. At a news conference in Damascus, Lt. Gen. Babacar Gaye, the head of the United Nations monitoring mission, said that it is escalating across the country and taking a toll on civilians. “It is clear that violence is increasing in many parts of Syria,” he said. “The indiscriminate use of heavy weapons by the government and targeted attacks by the opposition in urban centers are inflicting a heavy toll on innocent civilians.”

Gaye also said that about 50 observers, out of a team of 150, had withdrawn from the country, mainly because fighting in Aleppo prevented them from carrying out their work.

The Syrian military blasted at least half a dozen cities nationwide with artillery shells on Monday, killing approximately 93 people, according to the Local Coordination Committees (LCC), an opposition activist network.

A second video posted online and distributed by the LCC shows the interrogation of a man whom it identifies as the pilot, as three armed rebels stand behind him. “Our mission was to bomb the city of Mohassan today,” says the man being interviewed, who introduces himself as Mofeed Mohammed Suleiman.

When asked what message he would give other military officers working with the government, Suleiman replies, “I tell them to defect away from that gang.”

A rebel commander asks him about bruises on his face and the man, who appears relatively calm, says he sustained the injuries after ejecting from his jet.

The commander then says that the alleged pilot will be treated according to the Geneva Conventions, a sign that the rebels have understood that recent videos showing executions of Assad regime supporters have damaged their credibility.

Neither the video of the jet crash nor the one of the apparent interrogation could be independently verified.

Qassim Saaduldeen, an FSA commander in Homs, issued a statement on Monday that said the pilot was a resident of Homs whom he had known for 15 years. “We were working together in the military before I defected,” the statement said. “He is a genuine enemy of the revolution.”